1. Field of the Invention
This invention relates to caustic-resistant coatings for glass, and particularly such coatings which include a primer of a combination of an epoxy resin and an organofunctional silane. The invention has particularly valuable utility for providing clear, caustic resistant, adherent coatings for glass containers which strengthen the container and provide a safety factor.
2. Description of the Prior Art
Copolymers of alpha-olefins and alpha, betaethylenically unsaturated carboxylic acids when used as coatings for surfaces such as glass bottles, do not change the physical appearance of the bottles. The coatings exhibit good initial adhesion to glass, but the adhesion deteriorates rapidly in the presence of high humidity due to hydrophilicity. Beverage bottles of the returnable type are sterilized in a hot caustic solution (e.g., 5 percent NaOH, 70.degree. C.). One or two 15-minute caustic treatments have been found to destroy the adhesion between the glass and the polymers.
IN Belgian Pat. No. 822,689, issued May 28, 1975, improved adhesion between glass and ionic copolymers has been achieved by priming the glass with amino- or epoxy functional silane or epoxy resins. About five 15-minute caustic treatments can be tolerated before the adhesion between glass and the ionic copolymer deteriorates. Still further improvement in caustic resistance, e.g., 10 to 15 treatments, is achieved by overcoating the ionic copolymer layer with a polymeric, hydrophobic protective layer, e.g., nylon.
Wells U.S. Pat. No. 3,297,186 describes a method of permanently sealing glass surfaces together using an adhesive composition consisting essentially of a major portion of an epoxy resin, a curing agent and a minor amount of an amino-substituted alkylalkoxysilane. There is no teaching in this patent that a polymer or other nonsilicious material will form a strong caustic-resistent bond to glass via this adhesive.
Kiel U.S. Pat. No. 3,666,539 describes glass coatings having increased permanency of adhesion and capable of being rapidly cured at elevated temperatures. The coatings consist essentially of at least one carboxyl functional acrylic resin, at least one hydroxyl containing epoxy resin and a small effective amount of an ambifunctional epoxy reactive silane. The coating is said to exhibit permanency of adhesion to a glass surface when soaked in 3 percent by weight aqueous NaOH solution at 160.degree. F. (71.degree. C.) for a relatively short period of time. The acrylic compound acts as a curing agent, and the coating provides only poor protection against caustic.